1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
JP 2007-286627 A discloses a liquid crystal display device including a direct type backlight unit. In the liquid crystal display device, a plurality of light emitting diodes are used as light sources of the backlight unit. The light emitting diodes are disposed in matrix across an entire region of the backlight unit.
In the liquid crystal display device described in JP 2007-286627 A, the light emitting diodes are disposed across the entire region of the backlight unit, and hence the size of a substrate on which a large number of light emitting diodes are disposed needs to be large enough to cover the entire region of the backlight unit. This increases cost for preparing a large number of light emitting diodes as well as a material cost of the substrate on which the light emitting diodes are to be disposed.
To address the problem, it is conceivable to dispose the light emitting diodes in a concentrated manner in a region where the backlight unit is positioned, for example, in the vicinity of the center of the backlight unit in the short side direction, along the long side direction of the backlight unit so that light beams emitted from the light emitting diodes are reflected and diffused by an appropriate reflection sheet so as to irradiate an entire image formation region.
In this type of liquid crystal display device, in order to diffuse light from the light emitting diode, a lens for diffusing light beams needs to be provided in front of a light emitting diode element. The lens needs to be disposed to be slightly elevated from the substrate on which the light emitting diode element is mounted so as not to be contact with the light emitting diode element. Thus, a plurality of legs, for example, three legs are provided at a peripheral edge portion of the lens.
On the other hand, the plurality of light emitting diodes that are disposed along a specific direction in a concentrated manner are arrayed in line, and hence electrodes are formed on the substrate so as to connect anodes and cathodes of the light emitting diodes in series (hereinafter, this substrate is referred to as light emitting diode substrate). The electrode is formed so as to have as large area as possible in order to efficiently dissipate heat generated by the light emitting diode element.
FIG. 9 is a plan view illustrating an enlarged part of a plurality of light emitting diodes 13 arrayed on the light emitting diode substrate in line. In FIG. 9, portions covered by lenses 20 are illustrated by broken lines.
As illustrated in FIG. 9, a light emitting diode element 22 is disposed on a boundary separating two electrodes 23 from each other, and the anode and the cathode of the light emitting diode element 22 are connected to different electrodes 23. The lens 20 has a circular planar shape, and is disposed above the light emitting diode element 22 so that the center thereof coincides with a light emission region of the light emitting diode element 22. Note that, herein, the light emitting diode element 22 and the lens 20 paired therewith are collectively referred to as light emitting diode 13.
The lens 20 is fixed at a position slightly elevated from the light emitting diode substrate by means of a plurality of legs 21a, 21b, and 21c provided at the peripheral edge thereof so as not to contact with the light emitting diode element 22. In the example of FIG. 9, the three legs 21a, 21b, and 21c are provided, each of which is disposed so as to constitute the vertex of an equilateral triangle. The legs 21a, 21b, and 21c are each fixed to the light emitting diode substrate by bonding.
FIG. 10 is a partial cross-sectional view taken along the line X-X of FIG. 9. As illustrated in FIG. 10, the leg 21b of the lens 20 is fixed to the electrode 23 through the intermediation of a bonding layer 24b, whereas the leg 21a is directly fixed to the surface of the light emitting diode substrate 7 through the intermediation of a bonding layer 24a. In this case, the electrode 23 is a copper foil with a thickness of about 35 to 70 μm, for example. Therefore, the thicknesses of the bonding layers 24a and 24b are different by the thickness of the electrode 23. Returning to FIG. 9, the same is also applied to the leg 21c, and hence, in this example, only the bonding layer 24a for fixing the leg 21a has a non-uniform thickness and has a partially thick portion.
If the thickness of the bonding layer varies among the legs, the bonding strength varies among the legs. Accordingly, there is a fear that the bonding layer may peel off due to vibration or thermal stress applied to the lens 20 or the lens 20 may be fixed at an angle at the time of bonding, thereby causing brightness unevenness on a screen of the liquid crystal display device. In particular, it is better to increase the thickness of the electrode 23 in order to efficiently dissipate heat generated from the light emitting diode 13. It is expected, however, that the increased thickness of the electrode 23 leads to more unstable fixation of the lens 20.
As a possible countermeasure, as illustrated in FIG. 11, the electrode 23 is cut out at portions of fixing the legs 21a, 21b, and 21c of the lens 20 so that the legs 21a, 21b, and 21c are directly fixed to the light emitting diode substrate. This ensures that the lens 20 is fixed in a stable manner, but the area in which the electrode 23 is formed is reduced, thus impairing heat dissipation performance.